The present invention relates to a method of controlling hand over, in which timing of starting or ending hand over is judged by comparing a received level of a perch channel signal at a mobile station with a reference value, and, in particular, to a method of controlling hand over, which is suitable for controlling soft hand over in cellular mobile communication using Code Division Multiple Access (CDMA) system.
CDMA is one of the multiple access systems, and has advantages over Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) in simplicity of frequency allocation etc. Accordingly, CDMA is employed as a multiple access system in recent cellular mobile communication systems such as EIA/TIA IS-95 etc.
CDMA has a channel switching system called Soft Hand Over (SHO), which is unique to CDMA. SHO is realized when a mobile station communicates simultaneously with a plurality of base stations, and has an advantage that communication can be continued without a short break when a mobile station moves from a cell to another cell. Further, as disclosed in Japanese Unexamined Patent Laid-Open No. 8-18503, transmit power can be reduced since a mobile station can control a transmit power level of a traffic channel signal, depending on the base station having the best communication quality among the base stations simultaneously communicating with that mobile station.
In the following, will be described an outline of the conventional SHO, referring to FIG. 14.
In FIG. 14, boundaries C1, C2 that define ranges of cells covered by base stations 601, 602 are defined by received levels of perch channel signals transmitted all the time from the base stations 601, 602, respectively. Those perch channel signals are transmitted at a predetermined level of transmit power, usually modulated with the same frequency signal, and spread with different spread codes respectively for different base stations. The inside area of the boundary C1 on the side of the base station 601 is the cell range covered by the base station 601, and the inside area of the boundary C2 on the side of the base station 602 is the cell range covered by the base station 602. Now, will be considered the case in which a mobile station 50 communicating (through a traffic channel) with the base station 601 moves in the direction of the arrow A. When the mobile station 50 crosses the boundary C2, the mobile station 50 recognizes that it has entered the cell covered by the base station 602 based on the measurement result of the received level of the perch channel signal. Then, the mobile station 50 starts SHO, trying to establish communication with the base station 602. On the other hand, when the mobile station 50 crosses the boundary C1, it recognizes that it comes out of the cell covered by the base station 601 based on the measurement result of the received level of the perch channel signal. Then, the mobile station 50 ends SHO to disconnect communication with the base station 601, and communicates with the base station 602 only.